Much of the electrical power used in homes and businesses throughout the world is produced in power plants that burn a fossil fuel (i.e. coal, oil, or gas) in a boiler. The resulting hot exhaust gas (also sometimes termed “flue gas”) turns a gas turbine or boils water to produce steam, which turns a steam turbine, and the turbine cooperates with a generator to produce electrical power. The flue gas stream is subsequently passed through an air preheater, such as a rotating wheel heat exchanger that transfers heat from the flue gas to an incoming air stream, which thereafter flows to the combustor. The partially cooled flue gas is directed from the air preheater to the exhaust stack.
The flue gas contains contaminants such as sulfur oxides (SOx), nitrogen oxide (NOx), carbon monoxide (CO) and particulates of soot when coal is used as the primary fuel source. The discharge of all of these contaminates into the atmosphere is subject to federal and local regulations, which greatly restrict the levels of these flue gas components.
To meet the reduced levels of NOx emissions from power stations, as required by environmental regulations, many fossil fuel-fired electric generating units are being equipped with either selective catalytic reduction (SCR) or selective non-catalytic reduction (SNCR) technologies. In SCR, the most common method used is to inject ammonia or urea based reagents in the presence of a vanadium oxide catalyst where the ammonia reacts to reduce the oxides of nitrogen. SCR is generally accomplished at lower temperatures than SNCR. The SCR system typically operates at flue gas temperatures ranging between 300° C. and 450° C. U.S. Pat. No. 5,104,629 illustrates one known type of SCR installation.
In SNCR, the most common method used is to inject ammonia or urea based reagents into the upper furnace to reduce the oxides of nitrogen without the use of a catalyst. The SNCR system operates at flue gas temperatures ranging between 850° C. and 1150° C. U.S. Pat. Nos. 3,900,554, 4,208,386, and 4,325,924 illustrate known types of SNCR applications.
At coal-fired power plants, ammonia injection systems for SCR and SNCR systems are typically installed in the high-temperature and high-dust region of the flue gas stream, which typically is prior to ash collection. One disadvantage of current treatments for reducing nitrogen oxides in exhaust gases from stationary combustion sources is the large ammonia consumption.
There is a need in the art for alternative methods of reducing nitrogen oxides in exhaust from power plants and other stationary sources. The methods disclosed address that need.